Fix issue with negative weight in av1_jnt_convolve_2d_copy_sse2
Fix issue with negative weight in av1_jnt_convolve_2d_copy_sse2

BUG=aomedia:1573

Change-Id: Iebdec1f17be8f0f36bf3609854afb39cfd1bc1bb

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom